The present invention relates, in general, to battery systems and, more particularly, to monitoring and protecting rechargeable batteries.
Lithium-ion batteries are preferred over other types of rechargeable batteries such as nickel-cadmium batteries and nickel metal-hydride batteries for portable electronics applications because of their light weight and high energy density. However, lithium-ion batteries are very sensitive to overcharging, and safety is a major concern with their use. For example, a safety concern with lithium-ion batteries is that metallic lithium may plate onto one of the electrodes within the battery cell when it is overcharged. The plated lithium poses a fire hazard because of the flammable nature of metallic lithium. Another safety concern is the venting of noxious fumes when the temperature of the battery cell becomes too high. Furthermore, in an over-discharge condition, the voltage across a lithium-ion battery cell falls below an under-voltage limit, resulting in a change in the chemical composition of the electrolyte in the battery cell. Consequently, the life of the battery cell may be significantly shortened. Therefore, it is important to have a battery protection system that accurately monitors the lithium-ion batteries and ensures that they operate within their safe operating areas.
Conventionally, charging a lithium-ion battery requires a dedicated lithium battery charger. When the voltage of the lithium-ion battery being charged is significantly less than a fully charged voltage of the battery, the dedicated lithium battery charger operates in a constant current mode and charges the battery with a constant charging current. When the battery voltage is near the fully charged voltage of the battery, the dedicated lithium battery charger switches to a constant voltage mode. Under the constant voltage mode, the charging current flowing in the battery decreases exponentially as the battery voltage approaches the fully charged voltage, thereby preventing the battery from becoming overcharged. The dedicated lithium battery charger includes a charge control circuit that determines when the dedicated lithium battery charger switches from the constant current mode to the constant voltage mode, how much charging current flows through the battery during the constant voltage mode, and when the battery is fully charged. In order to charge the battery to its maximum capacity and effectively avoid overcharging, the charge control circuit is designed to be highly accurate. Typically, the voltage fluctuation of the charge control circuit is less than one percent (%).
The highly accurate charge control circuit significantly increases the cost of the dedicated lithium battery charger and, therefore, increases the cost of using a lithium-ion battery. Further, because the charging current flowing in the battery decreases exponentially as the battery voltage approaches the fully charged voltage in the constant voltage mode, the charging process is time inefficient. For example, a lithium battery is usually charged up to 80% of its capacity during constant current mode operation in a time interval ranging between one and two hours. Then, the dedicated lithium battery charger switches to the constant voltage mode and takes at least three more hours to charge the lithium battery to its full capacity.
Accordingly, it would be advantageous to have a battery protection system and a process for charging a battery. It is desirable for the system and the charging process to be cost efficient. It is also desirable for the charging process to be convenient and time efficient.